Antenna system



A. ALFORD ANTENNA SYSTEM Aug. 16, 193

Filed April 30, 1935 9 a w x W M I a W w W a w 2 x i p a E P FIG. 3

INVENTOR:

Ammw M0120 a, T ORNEV Patented Aug. 16, 1938 UNITED STATES ANTENNA SYSTEM Andrew Alford, New York, N. Y., assignor to Mackay Radio and Telegraph Company, New York, N. Y., a corporation of Delaware Application April 30,

5 Claims.

This invention relates to new and useful improvements in antenna systems and more particularly to unidirectional radio sending and receiving antennae.

5 According to the present invention a normal bidirectional antenna is limited to receiving or sending waves in one direction only, simply by inserting in it one or more phase delay devices intermediate its ends, an impedance matching device being preferably connected with the end of the antenna to prevent the reflection of waves therefrom. Owing to this arrangement waves travelling through the two or more sections of the antenna system divided by the phase delay devices will be retarded a predetermined interval of time, such that the radiations proceeding from or towards the various sections co-operate more effectively than heretofore practicable.

My invention, both as to its organization and 50 theory of operation, together with practical instructions by means of which one skilled in the art may obtain the beneficial results thereof, may best be understood by reference to the fol lowing description which is to be read in connection with the accompanying drawing in which:

Fig. 1 is a diagrammatical representation showing a short wave V shaped transmitting antenna of the travelling wave type incorporating my invention and arranged to facilitate an explanation of the underlying theory thereof;

Fig. 2 shows one method of constructing the delay device which is essential to my invention,

and; Fig. 3 illustrates an alternate method of constructing the delay device.

Consider first Fig. l in which I is a short Wave transmitter, 2 a transmission line similar in construction to an ordinary telephone line, which in turn is connected to a V shaped antenna of the travelling wave type, one side of which comprises the section 34 adjacent to the transmission line, the delay device 40, the section 55 which extends in the same direction as 34 and is of approximately equal length but on the far side of the delay device 48 and l is a terminating device for preventing reflection of waves from the outer end. Similarly, the other side comprises the section 8-9 adjacent to the transmission line, 90 the delay device, lllll the extended section, and l2 the terminating device for preventing reflection from the outer end. The delay elements Ga. and 90 may be old in strumentalities but their inclusion in the system of Fig. 1 when situated and proportioned in ac- 1935, Serial No. 18,995

cordance with my invention results in a substantial increase in efficiency of radiation in the desired direction over that obtained in the known types of travelling wave antennae employing equivalent radiators.

In order more fully to appreciate the role played by the delay devices it will be necessary to develop the fundamental theory along mathematical lines. This will show not only that there is latitude for improving the radiation efficiency but also the proper proportioning of the delay devices to secure the improvement.

In the first place, my invention is not applicable to antenna systems of the standin wave type but rather to -those of the traveling wave type. It is important to observe that with standing waves on an antenna wire the current and voltage are distributed along the wire in such a way that the maximum amplitudes vary sinusoidally along the wire. Furthermore, the maxima of these maximum amplitudes occur at fixed points at intervals of a half wave length along the wire, successive maxima being alternately in opposite phase. Between any two node points the oscillations are in phase. With travelling waves, while the current and voltage at any particular instant of time are distributed sinusoidally along the wire, there are no fixed points where nodes and anti-nodes occur, since at a later instant the curve of distribution of voltage and current moves bodily along the wire. Consequently, each element of length along the wire is capable of oscillating at the maximum amplitude, at given phase in the cycle of oscillations being reached for successive elements of length at later instants of time depending upon the length of wire comprising the successive elements and the velocity with which the waves are travelling. This difference between standing waves and travelling waves on an antenna gives rise to a difference in the respective radiation patterns and especially in the manner in which the radiations proceeding from the various sections of the wire cooperate at a distant point of reception. My invention consists in breaking up into sections of approximately equal. lengths the wires of the travelling wave antenna and introducing between successive sections a substantially non-radiating element whose function is to retard the travelling waves by a predetermined interval of time such that the radiations proceeding from the various sections of the wire cooperate far more effectively than heretofore.

n I: Z)

Referring to the radiating system of Fig. 1, it will be seen that each side is broken up into two approximately equal sections. It may be divided into any number of equal sections but two sections will suffice to explain the theory and operation. The sections of the lower side, for example, are 8-9 and l0ll With a delay device an connected between them. The point I9 is assumed to be a far distant point in the plane of the bi-sectcr of the antenna system. It is well known that the direction of maximum radia- I tion necessarily lies in this plane. Therefore, the practical consideration is how to enhance the radiation in the maximum direction.

First of all, the field strength at a distant point caused by radiation from a'straight wire is a function of the angle which a straight line drawn from the distant point to the wire forms with the wire itself and may be expressed by RM), where 0, is the angle and the expression as a whole is merely a compact way of representing the field strength. It is unnecessary in explaining the theory of my invention to treat this function -more explicitly. In the, light of the foregoing, I may express the field strength at the distant point .19 caused by radiation from section 89 by R109) and that from section ill-H by R2(0). (0), in each of these expressions is the same because both sections are in the same straight line. For the sake of analysis, it may be assumed that each section is replaced by a point oscillator having the same radiation characteristics as the section it replaces. Thus, for example, section 8--9 may be replaced by the point I3'and section Ill-II by the point Hi. If the length of each section is i, then in computing the combined effect of radiation from these sections, the distance between points i3 and H3 in the direction of the wire must also be equal to 1.

Points 15 and I6 are the projections of points l3 and M, respectively, on the arrow i8 which marks out the direction of maximum radiation in which I am mainly interested. This represents the distance'in the direction of maximum radiation which must be traversed by radiation from $3 in order to catch up with radiation from 54. Drawingthe line l5il parallel to l3|4, it will be seenirom the geometry of the figure that the distance l5'l6 is equal to, Z cos 0. Since electromagnetic waves in free space travel withthe ve-- locity of light, 0, the'extra distance l5l6 which must be traversed by radiation from E3 over that from M is equivalent to a time delay of 1 cos 0 T seconds.

The relative phases of the radiations from l3 and M at point 19 must be determined before the where it is assumed that the wave travels for all practical purposes with the speed of light. Therefore, the field strength at l9, due to the point oscillator l3, may be expressed by:

12 (9) Sin (tand that due to the point oscillator I 4, by:

R (0) sin w tg T) where w=21rf, f is the frequency and t the time. No appreciable error will be incurred by assuming R1=R2=R. Then the combined field strength at I 9, taking proper account of relative phase difference, is given by E,

e Let T be expressed in terms of a period of oscillation such that,

Regardless of whether the arrow It, representing the direction of maximum radiation for the antenna system, is in the plane of the wires of the antenna system or tilted at an angle thereto, as long as it lies in the vertical plane passed through the bi-sector of the angle between the two sides, the only factor directly affected by k is,

Obviously, this factor will have maximum values only-when,

or, expressed explicitly in terms of k,

In practice I prefer to use the expression,

because its use results in the most convenient designs. The following example is for the purpose of illustrating the proper method of applying this formula and, in general, the important considerations relating to the proper proportioning of the antenna system employing my invention.

Sections 3-4, 56, 39 and I 0-| I may, for example, be each a wavelength long. The travelling wave radiation pattern from a linear radiator a wavelength long is such that mximum field strength occurs at a distant point of reception when the direction in which radiation must be emitted by the radiator to be directed to the point of reception forms an angleof approximately 45 with the antenna wire. Therefore, with each section a wave length long I make the angle 0 of Fig. 1 equal to 45 degrees. At thisv point," it

should be noted that without my delay devices the angle would be determined by the radiation pattern of the entire length of each side of the antenna, i. e., in the case under consideration, for example, a total length of two wavelengths for which the angle 6 should be approximately 33 degrees. It is, therefore, apparent that breaking up the antenna sides into sections entails a distinct departure in the proper selection of the angle 0.

Substituting Z= and 0=45 in the equation,

it will be found that k=.707, i. e., .707 times the duration of one cycle of the radiated frequency. If I design the delay devices 40 and 9!! of Fig. 1 to effect a time delay of the value Ic:.70'7, then maximum radiation results in the desired direction. It may be shown that, with the same amount of energy fed to the antenna system in each case, the desired radiation secured with a V shaped antenna whose sides each have an overall. length of two wavelengths is substantially greater if each side is broken up into two sections of one wavelength each with the angle 0:45 degrees and a delay device in each side with Ic=.'707 as described, than it would be if each side consisted of an unbroken wire two wavelengths long with the angle 0:33".

I may choose sections of different electrical length in which case the procedure is the same as explained by the above example. That is, I determine the angle 0 on the basis of the radiation pattern of a linear radiator having the length of one section, and k by substituting the calculated value of 0 and the length of a section 1 in wavelengths in the formula for k.

The delay devices 40 and 90 may be designed in a variety of ways. One scheme is illustrated in Fig. 2 which consists merely of doubling back the antenna wire on itself, the length of the wire in the loop thus formed, i. e., the distance 20 to 2| to 22, being substantially equal to k times the wave length. For example, with a wavelength of 50 meters and k:.'707, the loop should be approximately 35.35 meters long.

Fig. 3 shows another arrangement for a delay device comprising two equal solenoids 23 and 24 whose axial lengths are much greater than their winding diameters. They may be conveniently wound with the same conductors used in the construction of the antenna. The delay device may consist of a single solenoid if its surge impedance is equal to that of the antenna wires to which it is connected. I prefer to construct two approximately equal solenoids as described and adjust the distance between the axes until the surge impedance of the parallel combination is equal to that of the antenna wires. It is well known that solenoids may be designed within practical limits for any desired delay, the required amount of delay in practicing my invention being given by the formula for k as explained.

What I claim is:

1. In an antenna system, a transmission line having two limbs, an antenna wire connected with each limb, each of the two antenna wires forming an acute angle with respect to a center line between them, an impedance connected with the end of each wire to minimize reflection of waves therefrom, and phase delay means inserted in each of said wires intermediate its ends and dividing said wire into sections, said delay means having such characteristics as to cause the energy radiated by said sections to add in phase in that direction in which each of said sections, considered individually, has substantially a maximum radiation.

In an antenna system, a transmission line having two limbs, an antenna wire connected with each limb, each of the two antenna wires forming an acute angle with respect to a center line between them, an impedance insulated from ground and connected with the end of each wire to minimize the reflection of waves therefrom, and impedance means inserted in series in each of said wires intermediate its ends and dividing said wire into sections, said impedance means having such characteristics as tocause the energy radiated by said sections to add in phase in that direction in which each of said sections, considered individually, has substantially a maximum radiation.

3. In an antenna system, a transmission line, an antenna wire connected therewith, an impedance connected with the end of the wire to minimize reflection of waves therefrom, and an inductance connected in series with the wire and dividing it into two substantially equal sections, said inductance having such value as to cause the energy radiated by said sections to add in phase in the direction in which each of said sections, considered individually, has substantially a maximum radiation.

4. In an antenna system, a transmission line, an antenna wire connected therewith, an impedance connected with the end of the wire to minimize the reflection of Waves therefrom, and a phase delay device having an impedance substantially equalling the surge impedance of the wire inserted in the wire intermediate its ends and dividing said wire into two sections, said delay device having such characteristics as to cause the energy radiated by said sections to add in phase in that direction in which each of said sections, considered individually, has substantially a maximum radiation.

5. In an antenna system, a transmission line having two limbs, an antenna wire connected with each limb, each of the two antenna wires forming an acute angle with respect to a center line between them, an impedance connected with the end of each wire to minimize the reflection of waves therefrom, and a phase delay device inserted in each wire intermediate its ends and substantially matching the surge impedance of the wire, said delay device having such characteristics as to cause the energy radiated by said sections to add in phase in that direction in which each of said sections, considered individually, has substantially a maximum radiation.

ANDREW ALFORD. 

